File format

Hi,
thank you for this great module.
I have just one problem:
when i try to open the file genereted by the module, Microsoft Office Excel 2007 send me this message (translated from Italian to English):
"The format of the file you are trying to open, 'filename.xls' is different from that specified by the file extension. Before opening the file, verify that it is not damaged and that comes from a trusted source. Open the file now? "[Yes] [No] [?]"

If i press "Yes" I can read the file anyway, but this alert is really annoying.
What could it be?

Regards